This invention relates to coating methods and particularly to coating horizontally aligned moving metallic strips.
More specifically, this invention relates to applying relatively thin coatings to both sides of a moving horizontally aligned metallic strip and to drying the coatings thereon.
Even more specifically, this invention relates to applying and drying thin coatings of photosensitive material to both sides of a metallic strip to eventually be utilized as aperture masks in color television cathode ray tubes.
As it is known in the art, most color television cathode ray tubes employ an apertured metallic mask positioned within the tube to aid in directing the electron beams emitted from the tube's electron guns in striking the tube screen. These beams excite in a pre-established manner the pattern of cathodoluminescent materials positioned on the screen to produce a desired color. A typical apertured mask contains hundreds of thousands of openings therein for this purpose. As can be appreciated, producing masks of this variety is a highly complicated and precision demanding process.
Previous methods for manufacturing apertured masks for color television tubes have substantially included coating a thin metallic strip with a photosensitive material, drying the material thereon, photoprinting a desired pattern on the strip, removing selective portions of the coating from the strip, and thereafter subjecting the strip to an etching step whereby the unprotected metal surfaces are etched through.
Heretofore, the application and dryig of the photosensitive coatings on the metallic strip have been achieved while the strip was in a substantially vertical position. As described in detail in U.S. Pat. No. 2,791,514 the coating material is applied in liquid form to the top marginal edge of the thoroughly cleaned metallic web and allowed to flow downward freely over both faces of the webbing. This is accomplished as the metal webbing travels edgewise through a coating station.
While this and similar methods have resulted in a coating being applied to both faces of a metallic strip, the uniformity of thicknesses of these coatings across the entire width of the strip has remained questionable. On far too many occasions, the coating thickness at the bottom of the strip proved greater than that at the top, most usually as a result of thicker concentrations of material accumulating at the bottom and not being able to dry at a sufficient rate with those along the upper surfaces. As can be appreciated, in precisely defined articles such as aperture masks, this difference in thicknesses can result in misalignment of the photoprinters during the photoprinting phase to thereby produce an uneven pattern of exposed or unexposed areas. Additionally, nonuniform thicknesses of the coatings can result in unequal rates of etching of the metal during the etching phase, therby resulting in some openings being larger than others.
It is believed, therefore, that a method for applying relatively thin uniform coatings to both sides of a moving substantially horizontally aligned metallic strip which would overcome the above cited disadvantages of prior art methods would constitute an advancement in the art.